This invention relates to nonreflective coatings, and more particularly to nonreflective coatings applied to optical elements such as lenses.
A nonreflective coating based on the interference of equal inclination is well known in the art, and such a nonreflective coating is applied to optical elements such as lenses (hereinafter referred to as "optical substrates" or simply "substrates" when applicable). The nonreflective coating of this type comprises three layers of optical films, or simply films (as in U.S. Pat. No. 3,185,020, and Japanese Patent Publication No. 28821/1975), and is disadvantageous in the following points.
(1) It is necessary to provide three different films which have high, middle, and low refractive indexes, respectively, on a substrate, and (2) furthermore it is necessary to change the refractive indexes of the three films, especially the refractive index of the film with the middle refractive index if the refractive index of the substrate is changed. The number in kind of materials which can satisfy these requirements is of the order of three if the reproducibility thereof is taken into consideration.
(3) The films are deposited, for instance, by vacuum evaporation. In this case, the film thus deposited is liable to be nonuniform in refractive index, and accordingly the reflection preventing effect thereof for the central part of the visible ray range is greatly deviated from the design value.
(4) It is very important in the field of photographical technique that such films show high transmissivity with respect to light in the near ultraviolet ray range. However, due to the above-described nonuniformity, a film made of a certain material may absorb a large part of the near ultraviolet rays at its thick portion.
In addition, (5) in order to eliminate the above-described nonuniformity, a method has been proposed in which gas such as oxygen or air is employed in depositing the films by vacuum evaporation. However, this method is disadvantageous in that the film formed thereby is insufficient in strength.